Conventional magnetic sensors for measuring current through a conductor include a magnetic circuit in the form of a core of magnetic material provided with an air-gap in which a magnetic field sensor, such as, for example, a Hall Effect sensor, is located. The conductor carrying the current to be measured, often called a “primary conductor”, passes through the magnetic circuit one or several times. Typically, such sensors are used in applications in which relatively large electrical currents are measured or in other applications in which simple devices, such as current shunts or transformers, are unsuitable for measuring the current to be measured, for example due to the need for galvanic isolation or the presence of D.C. components in the current.
Closed-loop magnetic sensors for measuring current flow through a conductor generally rely on the principle of current compensation in which the current generated by the primary conductor is compensated by means of current flow driven through a compensating coil, also known as a secondary coil, by an electrical circuit controlled by a magnetic field sensor placed in the magnetic core air-gap. In this way, magnetic sensor functions at the same operating point irrespective of the magnitude of the current being measured so that the non-linearity and temperature dependence of the magnetic sensor becomes irrelevant.
One example of a magnetic sensor for measuring current flow is a closed loop Hall current sensor. The Hall voltage is first highly amplified, and the amplifier's output current then flows through a compensation coil on the magnetic core. It generates a magnetization whose amplitude is the same but whose direction is opposite to that of the primary current conductor. The result is that the magnetic flux in the core is compensated to zero. Such Hall current sensors are sensitive to short current peaks in the circuit because the hysterisis properties of the magnetic core cause static magnetization in the core which results in a permanent remanence.
Another example of a magnetic sensor for measuring current flow is a closed loop Magnetoresistive current sensor. The Magentoresisitve sensor is also arranged in a magnetic circuit and the aforementioned compensation principle is used. In order to reduce the temperature dependence, the Magnetoresistive sensor is usually configured as a half or a full bridge. In one arm of the bridge, barber poles are placed in opposite directions above the two magnetoresistors, so that in the presence of a magnetic field the value of the first resistor increases and the value of the second decreases.
Closed loop current sensors of the aforementioned types suffer from several major drawbacks. Firstly, there is an inevitable offset current, associated with the effects of the magnetic core, whose amplitude and temperature dependence are subject to significant fluctuations. Secondly, non-linear properties of the magnetic core lead to a non-linear relationship between the primary conductor current, that is, the current being measured, and the compensating or secondary current, rather than a linear relationship more suited to current measurement.
A closed loop Magnetoresistive current sensor typically uses a ferrous core to concentrate the flux on to the magnetoresistive chip. There are many disadvantages of a ferrous core, such as for example, core losses, limited bandwidth, non-linearity, thermal instability, bulk and cost.
Regarding core losses, the magnetic material and core design as well as the current amplitude versus frequency spectra defines the level of core loss. Eddy current losses are proportional to the square of three different parameters: the peak flux density in the core, the frequency of induction and the lamination sheet thickness of the core. Hysteresis losses are proportional to frequency, core volume and the square of peak flux density. The core losses in conventional current sensors result in a temperature rise thereby further contributing to non-linearity and current off-set fluctuation. The bandwidth in a ferrous core is also limited by the core losses.
Yet another drawback of the aforementioned magnetic current sensors is that demand for such sensors using magnetic cores with larger cross sectional area is substantially increasing the dimension and weight of the magnetic sensor and the complexity and cost of the sensor manufacturing process.
There is a continuing need to provide magnetic sensor systems for current sensing, and/or other applications involving magnetic sensing, which are operable with improved linearity and stability. There is also a need to provide magnetic sensor systems which can be manufactured with greater ease and at lower cost.
The embodiments disclosed herein therefore directly address the shortcomings of present magnetic sensor systems providing a magnetic sensor system with improved performance and which is suitable for many price sensitive applications.